On the basis of the terminal fields of axons projecting from the A9 and A10-A8 mesencephalic dopaminergic cell groups, the striatum in the rat can be parcellated as three subdivisions: the nucleus accumbens and olfactory tubercle are innervated exclusively by A10-A8; the dorsolateral quadrant of the neostriatum is innervated exclusively by A9; a ventromedial part of the caudate-putamen (C-P) extending along its entire rostrocaudal axis is innervated by the A9 and the A10-A8 cell groups. Reported differences in the afferent innervation of the A9 and A10-A8 cell groups and differences in the striatal distribution of synapses from each, both at light and electron microscopic resolutions, magnifies the significance of the striatal region where the projections of A9 and A10-A8 overlap. A series of correlative light and electron microscopic investigations are proposed. In the first, it will be determined if the distribution of tyrosine hydroxylase-immunoreactive (TH-IR) boutons in the zone of overlap resembles the distribution in the nucleus accumbens or neostriatum, or represents a combination of more than one kind of synapse distribution. Complementary studies employing a combination of TH immunocytochemistry and Golgi impregnation will prove the distribution of TH-IR synapses on identified neurons in the ventromedial tier of the C-P. The anterogradely transported lectin, PHA-L, will be used at electron microscopic resolutions to demonstrate the kinds of synapses formed by projections originating in A9 and A10. A combination of tracing methods using the anterogradely transported lectin, PHA-L and wheat germ agglutinin, will be used to demonstrate the convergence of projections originating in A9 and A10 on unidentified neuronal elements in the ventral tier of the C-P. These studies will be supplemented by an investigation in which Golgi-impregnation, transported PHA-L, and anterograde degeneration will be combined to demonstrate the convergence of A9 and A10 projections on identified striatal neurons. Because the dopaminergic innervation of the posteromedial and anterolateral parts of the striatal domain of the olfactory tubercle is a potentially rich source of experimental information about dopaminergic and peptide influences on striatopallidal neurotransmission, these areas will be subjected to a battery of studies similar to those described above using antisera against TH, cholecystokinin, and neurotensin.